1. Field of the Invention
The present invention relates to a method for forming a semiconductor device using a crystalline semiconductor in a semiconductor device forming method. Further, the present invention relates to a method for forming thin film transistors (TFTs). TFTs according to the invention can be formed either on an insulating substrate as made of glass or on a semiconductor substrate as made of a single crystal of silicon. More particularly, the invention relates to TFTs formed, utilizing both a crystallization process using thermal and/or optical annealing and an activation process.
2. Description of Related Art
In recent years, an insulated gate semiconductor device comprising a thin film active layer (also called an active region) formed on an insulating substrate has been studied. Especially, researches on insulated gate thin film transistors (TFTs) have been earnestly conducted. These TFTs are formed on a transparent insulating substrate and used to control pixels of a display device such as a liquid crystal display having a matrix structure. Also, the TFTs are used in the driver circuit of the display device. Depending on the material and the state of the crystal of the used semiconductor, they are classified as amorphous silicon TFTs or crystalline silicon TFTs.
Generally, an amorphous semiconductors have small field mobilities and so they cannot be used in TFTs which are required to operate at high speeds. Since the field mobility of P-type amorphous silicon is quite small, it is impossible to form P-channel TFTs (PMOS TFTs). Therefore, a complementary MOS circuit (CMOS) which would be formed by combining an N-channel TFT (NMOS TFT) with such a P-channel TFT cannot be obtained.
On the other hand, crystalline semiconductors have higher field mobilities than those of amorphous semiconductors and thus can operate at higher speeds. With crystalline silicon, PMOS TFTs can be formed as well as NMOS TFTs and so CMOS circuits can be built. For example, in a known active matrix liquid crystal display, not only the active matrix portions but also the peripheral circuit such as drivers are composed of CMOS crystalline TFTs. This structure is known as a monolithic structure. For this reason, TFTs using crystalline silicon have been earnestly studied and developed recently.
One method of obtaining crystalline silicon is irradiation of an amorphous silicon with laser light or other equivalent intense light so as to crystallize the silicon. However, there is no prospect of mass production because of instability of the laser output and instability caused by the fact that the process is quite short.
The method which is currently considered to be capable of being put into practical use is to crystallize amorphous silicon by heat. In this method, crystalline silicon can be obtained with small variations among batches. However, this method is not free from problems.
Usually, in order to obtain crystalline silicon, it is necessary that annealing is carried out at about 600° C. for a long time or that annealing is carried out at a high temperature exceeding 1000° C. Where the latter method is adopted, the usable substrate material is limited to quartz, thus increasing the cost of the substrate greatly. Where the former method is adopted, the material of the substrate can be selected from various substances but shrinkage of the substrate caused during thermal annealing presents problems. In particular, a decrease in the forming yield due to misalignment of masks has been pointed out. Accordingly, there is a demand for a process at lower temperatures. Specifically, there is a demand for a process which is carried out below the strain points of (preferably at temperatures lower than the strain points of glasses by more than 50° C.) various non-alkali glasses used as materials of substrates. The present invention is intended to solve these difficulties. It is an object of the invention to provide a method of mass-producing TFTs without incurring the foregoing problems.
TFTs are formed using a thin film semiconductor formed on a substrate. These TFTs are used in various ICs. Especially, TFTs of this kind have concerned as switching devices located at pixels in an active matrix liquid crystal display and as driver devices formed in peripheral circuit portions.
It is easy to use an amorphous silicon film as thin film transistors used in TFTs. However, this method has the problem that the electrical characteristics are low. In order to improve the characteristics of TFTs, a crystalline thin film silicon may be used. Crystalline silicon films are variously known as polycrystalline silicon, polysilicon, and silicon crystallite. To obtain such a crystalline silicon film, an amorphous silicon film is first formed. Then, the film is crystallized by heating.
However, crystalline silicon thin films obtained by the conventional heating process have relatively small particle diameters, and these particles are not uniform in size. In consequence, their characteristics are not uniform. Furthermore, their mobilities which represents the performance of completed devices are much inferior to the mobilities of single crystal silicon. Therefore, there is a demand for a crystalline silicon thin film having improved characteristics.
Our research has revealed that crystallization can be performed at 450 to 650° C., e.g., about 550° C., in a short time on the order of 4 hours by depositing a trace amount of elements such as nickel, palladium, and lead on the surface of an amorphous silicon film and then heating the laminate. Also, the obtained crystal grains can be controlled by the temperature and time of the crystallization. This means that an active layer necessary for devices can be formed.
In order to introduce a trace amount of element as described above, or a catalytic element for promoting crystallization, plasma processing, evaporation, or ion implantation is employed. The plasma processing uses a parallel plate type or positive column type CVD apparatus. Electrodes containing a catalytic element are used. A plasma is generated in an ambient of nitrogen, hydrogen, or the like. In this way, the catalytic element is added to the amorphous silicon film.
However, if the above described element exists in abundance in a semiconductor, then the reliability and the electrical stability of an device using this semiconductor is deteriorated. This produces undesirable results.
In particular, an element for promoting crystallization such as nickel (referred to herein as a catalytic element) is necessary to crystallize amorphous silicon but it is desired that the amount of catalytic elements in the crystallized silicon should be reduced to a minimum. To fit this requirement, a catalytic element which tends to be inactive within crystalline silicon is selected. At the same time, the amount of a catalytic element necessary for crystallization is minimized. For this purpose, it is necessary to precisely control the amount of the introduced catalytic element.
Using nickel as a catalytic element, an amorphous silicon film is formed, nickel is introduced by plasma processing and a crystalline silicon film is formed by heating. The crystallization process is carefully examined and discovered the following items:    (1) Where nickel was introduced into the amorphous silicon film by plasma processing, nickel atoms are penetrated considerably deep into the amorphous silicon film before the heating processing.    (2) At first, nuclei of crystals are produced at the surface through which nickel atoms are introduced.    (3) Where a nickel film is formed on the amorphous silicon film by evaporation, crystallization occurs in the same way as in the case in which plasma processing is carried out.
From the above items, it is concluded that all the nickel atoms introduced by plasma processing does not function effectively. That is, if a large amount of nickel is introduced, some nickel atoms may not function sufficiently. Therefore, it is considered that the points or surfaces at which nickel atoms are in contact with silicon atoms function during low temperature crystallization. it is concluded that nickel atoms are required to be dispersed most finely, i.e., almost on an atomic scale. In other words, the requirement is that a minimum concentration of nickel is dispersed on an atomic scale near the surface of an amorphous silicon film within a concentration range which permits low temperature crystallization.
Evaporation can be used as a method of introducing an infinitesimal amount of nickel only into a surface region of an amorphous silicon film, i.e., introducing an infinitesimal amount of a catalytic element for promoting crystallization of an amorphous silicon film only into a surface region of the amorphous silicon film. However, it is not easy to control the evaporation process, and it is difficult to strictly control the amount of the introduced catalytic element.
It is necessary that the amount of the introduced catalytic element is reduced to a minimum. In this case, satisfactory crystallinity cannot be obtained.